Transformer connection for protective devices



Oct. 5, 1943. E. HARDER 2,331,136

TRANSFORMER CONNECTION FOR PROTECTIVE DEVICES Filed May a, 1941 INVENTOR 5am Emmi/ma:

ATTORNEY Patented Oct. 5, 1943 TRANSFORMER CONNECTION FOR PROTECTIVE DEVICES Edwin L. Harder, Forest Hills, Pa.,

Westinghouse assignor to Electric & Manufacturing Company, East Pittsburgh, Pa., a corporation of Pennsylvania Application May 3, 1941, Serial No. 391,653

7 Claims;

My invention relates to protective devices for protecting a multi-terminal bus or other electrical aparatus against internal faults therein, and it has particular relation to the parallel connection of voltage-producing couplers or ourrent-transformers which are substantially linearly responsive to the currents in the respective terminals of the bus or other protected electrical apparatus,

In a previous application, Serial No. 202,015, filed April 14, 1938, I showed a protective relay system, for bus-protection and other purposes, utilizing toroidal current-transformers which are both substantially linearly responsive to the terminal current, and substantially astatic or non-responsive to external influences. These toroidal current-transformers produce results difiering from ordinary iron-core current-transformers in producing an internal voltage, which is substantially linearly responsive to the linecurrent, rather than a secondary current which reflects the phase and magnitude of theprimary current, regardless of the secondary impedance. These toroidal current-transformers overcome a limitation of the iron-core type of current-transformer, which falls to properly reproduce the primary current under fault conditions, because of the loss of linearity of response which is suffered as a result of saturation in the iron cores of that type of current-transformers.

The toroidal current-transformers are useful," not only to avoid the distorting effects of ironcore saturation, which mismatch a plurality of ordinary current-transformers which are utilized to summate the terminal-currents of amultiterminal bus, but they are also useful to provide a convenient means for obtaining a response to the sum of a large number of terminal-currents. Since the output of a toroidal currenttransformer is a voltage, rather than a current, these toroidal transformers, when heretofore utilized for multi-terminal bus-protection, have had-their secondary circuits connected in series-circuit relation, instead of the familiar parallel-circuit connection which is utilized with current-producing current-transformers. The series connection of the toroidal current-transformers has one disadvantage, however, in case one of the current-transformers should become open-circuited, in which case the entire bus-protective i system would fail.

It is an object of my present invention to provide means whereby a plurality of these toroidal current-transformers, or other linear couplers,

which are associated with a plurality of ter- 5 minals of a bus or other electrical apparatus to be protected, may have their secondary circuits connected in parallel, and so arranged that the open-clrcuiting of any one of the parallel-connected secondary circuits (at least under favorable conditions, particularly Where the number of terminals or feeders is large) Will not cause a faulty relaying-operation,

With the foregoing and other objects in View, nay-invention consists in the apparatus, circuits, combinations and methods hereinafter described and claimed, and illustrated in the accompanying drawing wherein:

Figure 1 is a diagrammatic view of circuits and apparatus embodying my invention in a pre ferred form, and g Figs. 2 andB are detail-views showing difierent forms of the toroidal current-transformers or L near couplers which are diagrammatically represented in Fig. 1.

In Figure 1, I illustrate my invention as being applied to the protection of a multi-terminal alternating-current bus, which is marked Bus in the drawing. The bus may have any number of terminals. or connections through which current is fed into, or out of, thebus; the terminal-current vectors I1, I2, etc., being considered .to be positive if the current is flowing into the bus, in each case. In the illustrative form of my invention which is shown in Fig. 1, the bus has five terminals, which are represented by five feeders, l, 2, 3, 4 and 5, some of which will be load-feeders, while others will be generator-feed ers or feeders through which power is fed into the bus. My invention relates to an alternatingcurrent bus or other electrical apparatus to be protected, and this bus or protected apparatus may be either single-phase or polyphase. Hence, 1 may be regarded as a single-line diagram, representative of a polyphase system; or it may be regarded as showing only one phase-conductor of either a single-phase or a polyphase system, as Will be readily understood by those skilled in the art. Furthermore, by the expression "terminal, I mean to refer to any place or connection where current is led into or out of the bus, whether that current is a single-phase current or a polyphase current.

Each of the bus-terminals or feeders I to 5, or Whatever number there are, is provided with its own circuit-breaker OBI, CB2, etc., each having its own trip-coil Tl, T2, etc., which may be controlled, for feeder-protection, in any suitable manner (not shown) designed for the opening of any individual feeder-circuit in case of a fault understood that by the opening of all of the circuit-breakers CBi,

CB2, etc., in all the terminals for .feedeiis 0 5 which are connected to the bus, and my present invention is directed to the which controls the simultaneous tripping or opening of all of the terminal circuit-breakers CBL,

CB2, etc. This is done by vectorially adding, or summating all of the terminal-currents of the bus, and obtaining a response toa predetermined departure from the normal, or fault-free, condition in which the sum of the terminal-currents is zero. I

In carrying out my invention, .1 interlink ith eachfeeder or terminal I. toy-5, a. line up r, or voltage-producing current-responsive device, which is diagrammatically represented, in Fig. y a ecodary winding or l havin a mutual reactance M and a secondary impedance Z. In actual-practice, toroidal, current transformers areutilized, as shown in 2 and 3. These linear transforming devices operate by inducing a voltage 11M, 12M, etc which, in ach case, is a function of the terminal-current 11,1 etc,, mul-- tiplied by an impedance, M, said. impedance M being substantially constant for all obtainable currentevalues, and being for all of the terminals I to 5, her of terminals. there are. The internal, current-responsive, voltages 11 1 m, which are generated or induced in the secondary circuits of the several linearly responsivefcurrenttransfo mers, produce s condary rr n s which are in i ted at '1; i2, 9.-

n accordan e with m nvent on t e s a secondary windings MZxare utilized as paral lel-connected voltage-sources, of different voltage-magnitudesand phases dependent upon the relative "magnitudes and phases of the respective terminal-currents I1, 12, etc, to supply relayingenergy, to a single-phase output-terminal or measuring-circuit terminal, indicated by the rel g buses .20 a d 2 hi h sup ly Power to one or more fault-resp nsive relays or meteringdevices 22, or other voltage-energized currentresponsive means having a current I and an impedance Zm. Fig. 1,, the relay 2;? is diagrammatically represented .as .a multi-contact contactor which energizes the several tripping-circuits to each of the trip-coils Ti, T12, etc; althoug it wi be adi y the relay 2 i h ha e, been only a light-weight auxiliary relay which controls a larger tripping relay for that purpose,

Since the parallel-connected linear couplers or current-transformers MZ generate current-re.- spon-sive voltages, rather than current-responsive currents, the parallel connection of these cur.- rent-responsive voltage-sources. will not produce, at the terminals 28 and Z]. of the relay 22., ameasuringcircuit voltagewhich is properly reor whatever numsponsiye to the vectorial sum of all of the termirial-currents Ii, I2, etc, unless the secondary circuits of the linearly responsive voltage-producing current-transformers all have the same impede ance,-which I have diagrammatically indicated,

in F g. 1, by the impedance Z. These secondary bus-protective means su stan ially e am In the, form of invention. shown in a,

circuits consist of the secondary coils of the responsive current-transformers indicated at M Z, each of which is connected to the measuringterminals 2E! and 2! by means of electrical conductors 23 and 2 3 which, together with the secondary winding MZ, constitute a secondary circuit or a connecting-circuit between each of the linear couplers M and the output-terminals or measuring-circuit terminals 26 and M of the relaying system. The impedance Z of each or" these several connecting-circuits will usually be made up principally or altogether of the secondary impedance Z of the current-transformer, said impedance Z being composed of a self-inductance quantity, X and a resistance-component R. In actual practice, an auxiliary adjusting-resistance 25, and in some cases, also an auxiliary-adjusting inductance or reactance 25, may be included in each of the secondary-circuits for the purpose of exactly matching the total iinpedances Z of each of, these circuits against each other.

In rnyimproved relaying system, as just described, and as shown in Fig. 1, in the event of a fault on the bus, as indicated at X, the relayingcurrent Im in the fault-responsive relay or measuring-circuit 22 is directly proportional to the bus-fault current If, regardless of the distribution of this fault-current I: among the several bus-terminals." Designating the several terminal-currents, flowing into the bus, as 11, I2, 13 In, and designating the coi'resp0nding secondary-currents as i1, i2, i3 in, the voltageequations of the various secondary circuits, for the n terminals, are

etc. Adding, for an n-terminalbus,

Substituting the current-values,

in which all of the quantities are vector quan tities.

Equation 6 proves that, at the fundamental frequency, the relay-current In is proportional to the bus-fault current Ir, the constant of proportionality ein Z +nZ relay-amperes per fault-ampere. The denominator (Z+T7,Zm) 'may have any power-factor, either lagging, leading, or unity. If said denominator has a lagging reactanceecornponent XD which is large in comparison to its'resistance Rn, then the numerator lvl and the denominator (Z-i-nZm) will both be largely inductive reactances, and the response Z-l-nZ will be, substantially independent of the frequency, no more for the harmonics than for the fundamental frequency of the alternating-current system; Whereas, for direct-current, the numerator ,M becomes Zero and the denominator (Z+nZm) becomes a pure resistance, andthe response to the D.-C. transient-component of the fault-current is very small. a

With present manufacturing-method for the quantity-production of the linear couplers, it is comparatively easy to maintain a manufacturing-ac curacy of :2 in the uniformity of the various values of M in each of the n circuits, and accuracie of a constancy within-:1%are commercially feasible; but, at'present, accuracies of i0.25% do not seem to'be economically justified. That this is so, may be shown by assuming that an external fault-current, -Ir, is flowing in a terminal in which the mutual coupling is 1.01M, while the sum of the other[('n-1) terminal-currents is Ir, with mutual couplings of 0.99M in each. Then, instead of Equations 2 and 5, the sum of the voltage-equations will be -Ir(1.01M) +Ir(0.99M) +Im(Z+7LZm) (7) whence For an external fault, therefore, that is, a fault outside of the protected bus, with a through-current Ir flowing through the protected bus, the constant of proportionality is for the worst possible condition of unbalance of the mutual inductances M, with manufacturing accuracies of i1%. Comparing this quantity with the internal-fault response, where the constant of proportionality was Z +nZ 7.

it will be seen that the fault-responsive relay is only 2% as sensitive to external faults as to internal faults, under the extreme conditions of unbalance of th linear current-transformers. 1 It is usually quite feasible to give the fault-responsive relay 9. setting high enough so that it will not respond to 2%, or one-fiftieth, of the highest expectable through-fault current. In case -where greater than 25-to-1 ratio of maximum throughfault current to necessary relay-setting is encountered, it appears more economical and safe to use a restraint-winding on the relay, energized as illustrated in my application 202,015,

rather than to carry the manufacturingtolerance below i1%. Note that this 25-to-1 ratio, which I am'permitting, with iii-1% coil-tolerance, contemplates a 100% factor of safety.

The accuracy of my fault-detecting system also depends upon theaccuracy with which the total irnpedances Z of the connecting-circuits are kept identical. Since the mutual inductances', M, of the several coupling-transformers, are designed so as to be substantially identical, the self-inductances, X, of the same can also be made substantially identical, by using the same number 01 secondary turns, because M=N1NlP 9) and where N1 is the total number of primary turns or interlinkages of the toroid (being unity in the illustrated case), N2 is the total number of secondary turns of the winding on the toroid, and P is the total permeance of the flux-path of the toroid. Since the inductance of the two wires 23 and 24 joining the toroidal current-transformer M-Z to the parallel connecting-points, Or the measuring-voltage output-terminals of the entire series of summated current-responsive devices, is quite negligible as compared to the self-inductance X of the current-transformer, this inductance X may be regarded as representing the entire reactive part of the total impedance Z of each of the connecting-circuits. Any discrepan cies between the actual inductances X of the several connecting-circuits may be compensated for, if necessary, by adjustments of the small variable inductors 26.

The resistances R of the several impedances Z of the connecting-circuits can be made identical as by adjustments of the small variable resistors 25), but this may be done only for one particular operating-temperature, assuming that the toroidal current-transformers are wound with copper conductors, or with some other conducting material having a material or substantial temperature-coeflicient of conductivity. It is pertinent to inquire, therefore, what effect a temperature-difference of, say C. might have on the tendency to operate the fault-responsive relay 22 on "through faults.

Assume, for example, that half of the toroidal current-transformers are operating at 20 C., and the other half at C., and that a through fault-current, Ir, is coming in over the low-temperature transformers, each having a resistance of R and going out over the high-temperature ones, each having a resistance of 234.5+50 M (for copper coils).

The effects of temperature-difference in the impedances Z are obviously less when the inductances X of the connecting-circuits are large in comparison to the resistances R, and the circuits can and should be made largely reactive whenever feasible. Where the available spaces for the coupling-transformers are limited, however, it may be necessary to utilize a value of X which is equal to R or which may be even less than R in extreme cases. Thus, the ratio of the hot-coil impedance Z11 to the cold-coil impedance Zis Z \/Y +(1.12R) 5 25419; I] -2- X2+ R2 X2+ R2 If i is large with respect to R, the temperatureresponsive increase cally no difference in the impedance Z.

Assuming, however, a case in which 4Y=R, in

the impedance Z =R+a'i then the increment 0.12R in the resistance-component, as a result of a 30 heating, will be only 0.12( or 0.085, times the cold impedance 2. Equation 2 then becomes IVI(I1+I2+I3+ +11) 0.085Z(i2+i4+ For a through fault,

I1+I2+I3+ 20 (13) Substituting the value of the measuring-coil current Im from Equation 4, Equation 12 becomes Z.Im+0.085Z(iz+i4+ +1") +nImZm=0 (14) Also, adding every alternate equation of the group of Equations 1, substituting the value of the hotcoil impedance in resistance makes practiis set to respond is net obtai-ned by putting X=R in Equation 11, and remembering that the through-fault current is since'this matches the internal voltage-drops in the network-impedance with the external voltage-drop in the usefulload impedance Zm, and gives the maximum energy-consumption in the relay 22 for a given internal induced-voltage IrM upon the occurrence .of a fault on the protected bus.

Substituting Equation 20 in Equationsfi and 19, we obtain for an external orv through-current fault with the worst possible case of unbalanced heating of the current-transformers. This shows that, at the worst, a 30 temperature-differential would make the relay 8% as sensitive to through-faults as to bus-faults. If the ratio of maximum expectable through-fault current to the minimum expectable bus-fault current for which the relay over or about l2-to-l, this worst possible temperaturediiferential would not cause faulty tripping.

An important feature of my invention is that the open-circuiting of any one secondary circuit will not cause tripping under the maximum loadconditions, if the relay is set at a current-value greater than the maximum load-current, IL, in any one feeder or bus-terminal. The worst possible condition would be when the secondary circuit on this feeder becomes open-circuited while the load-current In is flowing in that feeder, there being no fault on the bus. Equation 2 will then become for an internal fault, and

whence Z 111 Under the optimum-energy design-condition of Equation 20, and-making thev approximating assumption that Zm hasthe same R/X ratio as Z,

Equation 24 becomes v m? 21L-12Z Comparison of Equation 25 with Equation 21 will show that the relay would have to be set at 2nl or more, to avoid the possibility of tripping on the maximum load-current IL with one secondary open." For a IO-circuit bus, (neglecting the phase angle between Zn and Z), this. means a relay-setting corresponding to or more.

This ability of my relay to avoid a faulty operation as a result of the open-circuiting of a secondary or relay-circuit, provides an opportunity for the open-circuit to be discovered, in the next periodic test, or by suitable supervisory alarmineans, and is believed to be preferable to dumping or disconnecting the heavy ,load as a result of an open-circuit condition in the relaying-circuit.

At the same time, even though one of the secondary circuits is open-circuited, my fault-responsive relay will respond to a bus-fault, provided that the current fed into the bus through the remaining terminals is in excess of the'value corresponding to the minimum pick-up value of the relay 22. This is believed to be very desirable.

While the open-circuited condition exists in one of the secondary circuits, there is a danger of erroneously tripping the protected :bus for a fault elsewhere, but this will occur only if the-current through the circuit whose secondary is open is greater than '2 t times the trip-setting of the relay- On a bus with a greatmany radial feeders, there is a good chance of correct operation.

Fig; 1 also shows an illustrati eform of embodh merit ofra polar-type, or polarized, supervisory relay cc, whichhas a plmalityiof coils 31- to 3B, alternate coilsbeing wound in alternat polarity, so that, if they are equally energized, as would normallybe the case, they will oppose each other, in pairs. One of these supervisory-relay coils is. connected in series-circuit relation with each of the secondary connecting-circuits associated with the respective terminal-couplers: M, and a very small supervisory -directfcurrent, of the order of milliamperes, which is well below the pick-up point of the fault-responsive relay 22, is circulated through the respectivelinea-r couplersM, by means of a small battery 3? which is connected. across the metering bus.213-.2l through a current-limiting resistor 38. Incase the protected bus has an odd number of terminals, as in the illustrated example, the left-over supervisory-' relay coil 35 is energized through a dummy-resistance 39, so as to carry the same current it would'have carried if it had been connected in another secondary circuit of the relaying system.

By means of the-supervisory-relay connection just described, if'there should be any open-circuit condition or other faulty condition in any one of the secondary relaying circuitathe balanced condition between successive pairs-of the supervisoryrelay coils 3| to 36 will be disturbed, in one direction or the other, causing the movable spring-arm 40 of the supervisory relay 30 to move, in one direction or the other, and close one of the sets of contacts 4| or 42, so as to energize-a bell or other alarm 43 across th supervisory battery 31.

Most of the energy for operating the supervisory relay 3|] is obtained from its permanent magnet 44, or equivalent polarizing-means which is utilized to polarize the relay, and the amount of direct current which is carried by the coils 3| to 36 of this relay is so small, and the number of turns of the supervisory-relay coils 3| to 36 is so small, and on a magnetic circuit having so much air-gap therein, that the mutual coupling between these coils 3| to 35 is negligible in comparison with the rest of the impedance Z of the respective secondary circuits of my relaying system. The supervisory relay 30 also preferably has a damper-coil 45 which makes it insensitive to alternating currents.

The broad features of the particular form of supervisory relay 3!], in a somewhat different application, are described and claimed in a companion application of Myron A. Bostwick and Bert V. Hoard, Serial No. 278,845, filed June 13, 1939, Patent Number 2,276,150, and assigned to the Westinghouse Electric 8: Manufacturing Com- D y.

The particular form of linear coupler M-Z, which I prefer to utilize, is the toroidal currenttransformer which is described and claimed in my previously mentioned copending application Serial No. 202,015. Two exemplary forms of embodiment of this coupler are shown in Figs. 2 and 3, respectively.

In Fig. 2, the secondary winding 50 is formed with an air-core toroid, and the winding 50 is wound in a plurality of pairs of layers, although only two layers and 52 are illustrated, these layers progressing in opposite directions around the toroid, so that the second layer starts at the point where the first layer ends, and progresses backwardly around the toroid, overlying the first layer, so as to render the coil substantially astatic or unresponsive to magnetic fields or magnetic influences other than those circulating around within the toroid.

In Fig. 3, the toroidal core is made of an insulating material, as shown at 53, and is provided with a circularly extending peripheral slot 54 in which are placed the return-conductors 55 for the several coil-layers 56 and 51, each of these coil-layers traveling or progressing around the toroid in the same direction, in the process of winding the coil, the astatic properties being obtained by bringing back as many turns, in the reverse direction, by means of the return-conductors 55 which constitute as many turns in the backward direction as the forwardly progressing turns of the windings 55 and 51.

In either of the types of linear couplers shown in Figs. 2 and 3, it is feasible in quantity-production methods, to build coils which are sufficiently astatic.

I claim as my invention:

1. Summation current-responsive means for the detection of faults in a multi-terminal alter nating-current apparatus to be protected, comprising, in combination: a relay with only one winding-circuit; a plurality of voltage-producing current-responsive means adapted to be associated, one with each terminal of the apparatus to be protected, for deriving, in each case, a voltage which is a substantially identical predetermined function of a terminal-current of the apparatus times an impedance which is substantially constant for all obtainable current-values and which is substantially the same for all terminals; and a plurality of connecting-circuits associated, one with each voltage-producing current-responsive means, for connecting the several voltage producing current responsive means, as parallel-connected sources of supply, to the terminals of said winding-circuit, said connecting-circuits including means for causing them to have substantially identical total impedances.

2. The invention as defined in claim 1, characterized by the impedance of each of the connecting-circuits being of the order of n times the impedance of the voltage-energized current-responsive winding-circuit, where n is the number of terminals of the apparatus to be protected.

3. The invention as defined in claim 1, characterized by the total impedance of each of the connecting-circuits being largely reactive.

4. The invention as defined in claim 1, characterized by each voltage-producing current-responsive means being substantially astatic and producing an internal voltage which is substantially linearly responsive to the first derivative of a terminal-current of the appparatus to be protected.

5. The invention as defined in claim 1, characterized by each voltage-producing current-responsive means being an air-gap current-transformer having a sufi'icient amount of air-gap to make the transformer substantially free of saturation-efiects.

6. The invention as defined in claim 1, in combination with supervisory relay-means for responding to an open-circuit condition in any connecting-circuit or in the voltage-producing current-responsive means which is associated with any connecting circuit, said supervisory relay-means comprising a senitive direct-current relay having a, plurality of pairs of balanced coils on a common magnetic circuit, the coils being connected in different individual connecting-circuits, and means for supplying, to the terminals of said common, voltage-energized current-responsive winding-circuit, a unidirectional supervisory current too small to actuate the fault-responsive device.

7. A relaying system comprising, in combination: an alternating-current relaying-device having a common winding-circuit; a plurality of sources of alternating-current relaying-energy; a plurality of connecting-circuits associated, one with each source, for connecting the several sources, in parallel-circuit relation, to the terminals of said common winding-circuit, said connecting-circuits having substantially identical total resistances; a sensitive direct-current supervisory relay having a plurality of pairs of balanced coils on a common magnetic circuit, the coils being connected in different individual connecting-circuits; and means for supplying, to the terminals of said common winding-circuit, a unidirectional supervisory current too small to actuate the alternating-current relaying-device.

EDWIN L. HARDER. 

